The present invention relates to a radiation image reading system and a radiation image reading method which are appropriate for use in a medical radiographic system to obtain a radiation image, such as an x-ray image, penetrated through the chest of the human body or the like. More specifically, the present invention relates to a reading system and method in which a semiconductor detecting means for converting radiation photons penetrated through the subject into an electric signal is provided, and wherein when the radiation image of the subject is reproduced as a visual image, an image signal obtained from the semiconductor detecting means is processed so that graininess is improved in the low density area of the radiation image, while sharpness is maintained in the high density area.
In many cases, radiographic systems can be used for both medical and industrial applications. In the medical X-ray photographic system, such a radiation image processing apparatus is used that a storage phosphor material (photo-stimulable phosphor material) is used as an intermediate medium, an X-ray image is recorded therein, the X-ray image is read out and reproduced, and recorded in the recording material as a final image.
A technological document relating to this type of the radiation image processing apparatus has been disclosed in Japanese Patent Publication No. 62373/1987. In the disclosed radiation image processing apparatus, a storage phosphor is used for detecting the X-ray image signal. When an X-ray is irradiated onto the storage phosphor, energy corresponding to the exposure is accumulated therein, and when a light beam with a specific wavelength (a laser light, or the like) is irradiated, luminescence corresponding to the accumulated energy is emitted.
According to the X-ray image processing apparatus utilizing the storage phosphor, by irradiating the X-ray onto the storage phosphor through a subject, the X-ray image of the subject is accumulated in the storage phosphor for recording. Further, when an excitation light scans on the storage phosphor in which the X-ray image is recorded, the photo-stimulated luminescence, generated from the storage phosphor by the excitation light, is converged and then converted into an electric signal by means of a photo-electric conversion device such as a photomultiplier, etc.
An original image signal read from the storage phosphor is defined as xe2x80x9cSorgxe2x80x9d. The spatial frequency which is important for diagnosis exists in a spatial frequency area having a relatively low image density (hereinafter, referred to as the low spatial frequency component), though there are some amount of differences depending on each part of the human body. Consequently, unsharp mask processing is conducted such that the low spatial frequency component is emphasized so that the contrast is enhanced.
For example, an unsharp mask signal Sus corresponding to the ultra low spatial frequency component is obtained at each scanning point on the storage phosphor. Herein, when an emphasizing coefficient is xcex2 and a reproduction image signal is Sxe2x80x2, the emphasizing coefficient xcex2 is monotonously increased corresponding to an increase of values of an original image signal Sorg or the unsharp mask signal Sus, and the calculation is conducted by using the following arithmetic expression:
xe2x80x83Sxe2x80x2=Sorg+xcex2(Sorgxe2x88x92Sus)xe2x80x83xe2x80x83(1)
According to this calculation, the reproduction image signal Sxe2x80x2 can be obtained such that the low spatial frequency component and a higher spatial frequency component which relate to the X-ray image of the subject, are emphasized.
However, in the X-ray image reading system using the storage phosphor, a means for irradiating the excitation light beam onto the storage phosphor in which the X-ray image has been recorded, a means for scanning by using the excitation light beam, means for converging the stimulated luminescence generated from the storage phosphor and then converting it into an electric signal, and a means for erasing the remaining image, or other means prevent the X-ray detecting means and consequently the X-ray photographic system from being simplified and reduced in weight.
Accordingly, recently, an X-ray image reading system having a semiconductor detecting means for converting X-ray photons into an electric signal is being developed. In this type of X-ray image reading system, being different from the X-ray image reading system using the storage phosphor, it is not necessary for the present system to irradiate the excitation light such as laser light beams for reading the X-ray image information, thereby, a blur does not occur due to scattering or diffusion of the excitation light, and an X-ray image with a very high sharpness can be obtained. Further, a scanning system and an optical reading system of the excitation light, a mechanical conveying system, and an erasing system of remaining image are not necessary. Accordingly, in the X-ray image reading system having the semiconductor detecting means, a very sharp X-ray image can be obtained as compared with the X-ray image reading system using the storage phosphor, as well as the reduction of the size and weight of the X-ray photographic system can be achieved.
In the conventional X-ray image reading system having the semiconductor detecting means, however, there are many cases in which the graininess is deteriorated as the sharpness is increased. In the low density area in which a quantity of X-rays, which reaches the detector, is small, because originally the graininess is worse due to quantum noises as compared to that in the high density area in which a quantity of X-rays is large, deterioration of the graininess as an increase of the sharpness tends to be more conspicuous, and deterioration of the graininess in the low density area corresponding to the mediastinum or the abdomen of the image of the chest, onto which an exposure of X-rays is small, is specifically conspicuous.
In such a case, when unsharpness mask processing of the X-ray image reading system using the storage phosphor is applied to the X-ray image reading system using the semiconductor detecting means, and when the low spatial frequency component and the higher spatial frequency component are emphasized so that the contrast is enhanced, there occurs a problem in which the graininess is more conspicuous and the image quality is deteriorated in the low density area of the X-ray image.
Accordingly, the present invention solves the foregoing problems, and the object of the present invention is to provide a radiation image reading system and a radiation image reading method in which the graininess can be improved in the low density area of the radiation image, while the sharpness is maintained in the high density area thereof.
In order to achieve the abovementioned objectives, there is provided a radiation image reading apparatus which is comprised of:
a semiconductor detector for converting radiation photons penetrated through a subject into electric signals to generate a first radiation image information; and
a processor for processing the first radiation image information so that a modulation transfer function in a low density region is not higher than a modulation transfer function in a high density region, in order to generate a second radiation image information.
The low density area of the electric signal means a signal area in which a dosage irradiated onto (reached) the semiconductor detecting device is small. The high density area means the signal area in which a dosage irradiated onto (reached) the semiconductor detecting device is large.
Further, the low density area of the electric signal is the signal area not larger than an intermediate value between the maximum value and the minimum value of the signal area corresponding to a dosage of a portion to be used for the diagnosis in radioactive rays irradiated onto the semiconductor detecting device, and the signal area to be used for the diagnosis. In the same manner, the high density area of the electric signal is not smaller than the intermediate value, and the signal area to be used for the diagnosis.
The first image signal is the original image, and is the analog signal obtained by the semiconductor detecting means, the digital signal after the A/D conversion of the analog signal, the signal on which signal amplification processing or temperature conversion processing is conducted, or the like. A signal obtained after black and white reversal processing is conducted, may also be allowable.